There is a great need to treat defects in bone that result from either trauma, disease, or reconstructive surgery. Autologous bone or cadaver bone are the traditional graft materials used to replace bone in these circumstances; however, the use of these particular graft materials can increase the morbidity associated with the surgical management of the condition. As a result, synthetic materials are being developed to function as either bone grafts or templates containing isolated cells that foster bone regeneration. A major limitation with current materials used as artificial grafts or templates for tissue regeneration is the invasive procedures needed for placement of the device and the lack of designed interaction between the material and the isolated cells. This proposal details -the synthesis and characterization of loosely crosslinked Poly(N-isopropylacrylamideco-acrylic acid) hydrogels that are extremely pliable and fluid-like at room temperature (RT), but demonstrate a phase transition as the matrix warms from RT to body temperature, yielding more rigid structures. Thus, these hydrogels offer the benefit of in situ stabilization without the potential adverse effects of in situ polymerization (e.g., residual monomers, initiators, catalysts, etc.). We envision exploiting this thermoreversible behavior by incorporating autogenous cells (e.g., osteoblasts) into the hydrogel at room temperature when the matrix is pliable and the cells can be easily distributed throughout the matrix, and then allowing the phase behavior of the hydrogel to entrain the cells and stabilize when implanted. The hydrogels are also amenable to modification with biological ligands that interact with integrin cell surface receptors, such as the -RGD- and-FHRRIKA- signals previously identified in the P.I.'s laboratory. We aim to synthesize hydrogels that incorporate these biomimetic peptides that facilitate proliferation and differentiation of osteogenic cells (e.g., osteoblasts), and incorporate peptide crosslinkers that are cleaved by matrix metalloproteinases (e.g., MMP-13) synthesized by these cells. This application has three parts in which two major hypotheses will be tested. In Part 1 the investigators will characterize the physical, chemical, and degradation properties of these gels. In Part 2 the investigators will evaluate their ability to promote proliferation and phenotypic expression of either rat or human derived osteoblasts in vitro. In Part 3 they will assess the ability of the biomimetic and degradable hydrogels, seeded with human derived osteoblasts, to foster the formation of tissue with the histoarchitecture of bone in vivo, in an ectopic site in an athymic mouse model.